1. Field of the Invention
This invention relates to the reduction of aerodynamic drag for moving ground vehicles; specifically to an improved method and devices for the reduction of aerodynamic drag and for improved performance and stability of ground vehicles by stabilizing and increasing the mass and velocity of the airflow passing under a vehicle.
2. Description of the Related Art
There have been many attempts to reduce the aerodynamic drag of a tractor-trailer system with the assumption that there is a great amount of airflow in the area behind the tractor drive tires, between the undercarriage of the trailer and the roadway, and all the way to the trailer tire assembly. Because the trailer is connected behind the tractor, the tractor interrupts the airflow. A smooth stream-like flow does not exist but in fact it is comprised of an unsteady, chaotic nature moving along with the tractor-trailer components at a reduced speed compared to the tractor-trailer speed through the air with some momentary airflow components moving in a forward direction relative to the forward motion of the vehicle.
Many have taught that the best way to improve tractor-trailer aerodynamics is to block the air from entering under the trailer by installing side plates—Wood et al., U.S. Pub. No. 2011/0204677, 2012/0319428, 2009/0195017 and Heppel et al., U.S. Pub. No. 2009/0212595 for examples.
Others teach that aerodynamic shields can be used to gain aerodynamic improvements such as seen by FitzGerald, U.S. Pat. No. 4,640,541, Ortega et al., U.S. Pub. No. 2010/0066123 and McErlane, U.S. Pat. No. 4,262,953.
Still others teach that some other airstream deflectors as Domo et al. U.S. Pub. No. 2011/0068605 and Whitney et al., U.S. Pat. No. 4,486,046 can provide aerodynamic benefits by deflecting the air under the trailer. The problem is that the airflow found under a trailer is comparatively slow moving and very chaotic in nature. Without introducing any airflow straighteners or other flow enhancement techniques these devices are nearly worthless. All of the prior teachings fail to realize that the airflow under a truck trailer moving through the air at highway speeds is really comprised of an unsteady, unorganized flow and at reduced speeds compared to the vehicle's operational velocity.
All of these teachings assume that the airflow under the trailer will be moving past the components in the under-trailer area in some organized manner at some speed near the tractor-trailer road velocity. This is not the case. The chaotic, turbulent airflow in the region behind the tractor tires, between the roadway and trailer undercarriage and all the way back to some distance behind the trailer rear doors is moving much less then the tractor-trailer's road speed velocity. The air is being dragged along by the physical components of the tractor and trailer assemblies. Live over-the-road smoke testing reveals that the turbulent airflow under the trailer is so chaotic that it has some components that actually move briefly forward, towards the front of the vehicle. This fact dispels the notion that the airflow moves past the undercarriage area at the same speed as the vehicle speed. It also dispels the idea that the air is moving in an organized way. Instead of the under-trailer airflow creating huge aerodynamic drag components from positive pressure vectors developed by hitting undercarriage components only, there are drag vector components generated by negative aerodynamic pressures. These negative pressures under the trailer are very similar to the low-pressure area found at the rear of the trailer commonly called a vacuum area. This low-pressure area behind the tractor is increasing along with the development and implementation of newer aerodynamic tractor designs. These designs create low-pressure areas behind the tractor especially when they employ deep skirts along the lower outside edges of the tractor.
All of these teachings fail to take full advantage of their aerodynamic concepts because they add what can be referred to as device drag. The addition of the side-mounted skirts on the tractor and/or the trailer has the effect of increasing the flat plate drag area at the rear of these components. Shielding devices add negative pressure zones directly behind them on the lee side of the devices. The lee side of any shield cancels out a portion of the aerodynamic improvement gains that may occur by moving the air along their curved, up wind surfaces. Undercarriage airstream deflectors cannot do noticeable work without an organized airflow. If airstream deflector designs end in solid shapes they will contribute to chaotic airflows just as a flat plate is known to do.
All of these afore mentioned teachings relate some kind of perceived improvement for the trailer axle assembly area. None have consideration for the completely chaotic and turbulent airflow that actually exists under and behind the truck and the trailer as a whole unit. Therefore none of these teachings ever suggest that an improvement can be had if the mass and velocity of the airflow under the trailer is increased.
None of these teachings of under-trailer mounted devices provide a way to automatically move the device with the trailer axle assembly. The limited aerodynamic benefits that may occur are negated when the axle location is moved away from those devices. In fact, it has been found that when a trailer axle assembly is moved away from trailer side skirt deflectors the effect in a side wind is to create an increase in aerodynamic drag.
No consideration of said teachings provide a means to easily remove them for maintenance access or replacement of their devices should they become damaged in some accidental way.